So you just upgraded to the newest smart phone. When you go to pair it with your car it will not do so. What happened?! The example here is with an iPhone 6S and a 2013 Honda Civic.
The problem is with the phone name. In this case, the car was set up using the old phone, (with a different iOS), called “Captain’s” iPhone. When I changed the name of the new iPhone to Andrew Sr it worked perfectly and the Bluetooth was active again!
To change the name of the iPhone, plug it in to your computer. When iTunes opens, click on the name of the phone in the upper left corner. When highlighted in blue, type in the new name of the device right over the current one. Hit enter to lock in the name. Unplug the phone, plug it back in to resync the data files with the new name. Done.